A typical method for diagnosing a temperature sensor mounted in a vehicle utilizes temperatures of different sites of an engine in a thermal equilibrium state. Temperatures are measured with temperature sensors at the start-up of the engine after a long parking period. The difference between the measured temperatures outside a normal range indicates the presence of a fault in the diagnosed temperature sensor (See Japanese Patent No. 5,531,776).
“A thermal equilibrium state” here refers to the condition of the engine at substantially equal temperatures at different sites with substantially no or slight heat transfer between these sites. If several to about fifteen hours elapse after the shut-off of the engine (the elapsed time is referred to as a soak time or an engine shut-off time), the temperatures at the different sites of the engine approach the ambient temperature and thus these site are in a thermal equilibrium state. The measurement of temperature in a thermal equilibrium state is not affected by the draft cooling of the engine nor the heat generated in the engine while the vehicle is running. The comparison of the difference between the measured temperatures with the normal range enables a reliable diagnosis.
The methods for determining the thermal equilibrium state of an engine are categorized into measurement of the elapsed time between the shut-off of the engine and the start-up of the engine (first method) and comparison of the temperatures at different sites of the engine at the start-up of the engine (second method).
In the first method, the elapsed time after the shut-off of the engine is measured by a soak timer. When the elapsed time exceeds a predetermined time, the engine is determined to be in a thermal equilibrium state. For example, when the elapsed time is eight hours or more, the engine may be determined to be in a thermal equilibrium state. A vehicle without a soak timer operable during the shut-off time, however, cannot measure the elapsed time. The vehicle with the soak timer cannot obtain high-accuracy diagnostic results since the first method depends on a single fixed reference value. The time until the engine reaches a thermal equilibrium state varies depending on several conditions, such as temperatures at different sites of the shut-off engine (initial temperatures) and the ambient temperature during the shut-off time (a convergent temperature).
In the second method, temperatures at different sites of the engine are measured by temperature sensors at the start-up of the engine (at the turn-on of the main switch). A small difference between the measured temperatures determines that the engine is in a thermal equilibrium state. Both the temperature of cooling water for the engine measured with a water temperature sensor and the temperature of the engine oil measured with an oil temperature sensor are close to the ambient temperature after the engine reaches a thermal equilibrium state. A small difference between the water temperature and the oil temperature determines that the engine is in a thermal equilibrium state. This method can be applied to a vehicle without a soak timer.
If a vehicle is parked in bad weather in a cold region and is cooled rapidly, however, the water temperature and the oil temperature may drop to have a small difference therebetween before the engine reaches a thermal equilibrium state. As a result, the engine may be erroneously determined to be in a thermal equilibrium state even if the actual elapsed time from the shut-off of the engine is significantly short (tens of minutes, for example). The diagnosis of a temperature sensor based on the erroneous determination, using the measured temperatures affected by the heat of the engine, leads to inaccurate diagnostic results.